Unused constant APP_URL?

[sapphirecat]

I was trying to make sure I have everything in the right place on my site, and I realized I don't understand what APP_URL is. The only uses of it I could see (apart from creating it in setup.php) are when validating the config, checking that it is defined and not the empty string.

My understanding of IndieAuth and selfauth is that the USER_URL should be the Profile URL (my index, which is the page with <link rel="authorization_endpoint" ...> on it) and the APP_URL is a red herring.

Am I missing something? Thanks!